Two challenges that you will need to over come are drift of freq due to the extreme tempratures any of your radio equipment is exposed to.

You may get out of range of the GSM base once you reach an altitude above 5 miles. This could be improved by using a small yagi style antenna. DO not purchase one just make it form thick copper wire to keep weight down

Another solution is to use licensce free radio equip www.lprs.co.uk they supply some modules that you can connect directly to a gps rciever. The ground station could use a reciever with high gain Yagi antenna, and the data could be fed directly into a PC using google earth .

this has been done a few times already. The cheapest solution is a SMS based GPS tracker. Yes, it won't function above a certain altitude but then again you only need it when you retrieve the balloon. GPS itself has no altitude limitation (heck, the ISS is using it...)_________________Lutz

We have something - http://www.tdc.co.uk/index.php?key=lr8m03 - but bear in mind it is illegal to use GSM in an airborne situation - not because of interference with aircraft, but it is the terms of use of GSM - similar to most PMR licences.

This is mainly because your balloon would be able to 'see' GSM base stations for hundreds of miles once airborne! - and the networks will suffer interference._________________G1NTX AFRIN LRPS

And again, commercial GPS units will refuse to present a fix over the SPS height limit of 60,000 feet. You may well be able to recover psuedo-range information (I have not tested it...), but position will not (or at least should not) be returned. Export licences would not be granted for navigation equipment without such a restriction.

L2 receivers will work though at greater altitudes, they will not have a restriction._________________G1NTX AFRIN LRPS